The present invention relates to the field of wavefront sensors, and more particularly those employing shearing interferometers.
It is often desirable to measure the shape of the wavefront of an incoming beam of light by measuring the wavefront slope at a plurality of locations in the aperture of the beam being measured in two given directions. Shearing interferometers have been employed to perform this operation and reference may be made to U.S. Pat. No. 3,829,219 to James Wyant and U.S. Pat. No. 3,923,400 to John W. Hardy, together with a paper to J. F. Ebersole and James Wyant entitled "Collimated Light Acoustooptic Lateral Shearing Interferometer", Applied Optics, Volume 13, No. 5, May, 1974 and the references cited therein. See also copending patent application Ser. No. 562,656, filed Dec. 19, 1983 in the names of Al MacGovern and Richard Howell, entitled Shearing Interferometer Employing An Acousto-Optic Cell. The above mentioned publications teach the use of A.C. interferometers which employ moving gratings of different spatial frequencies to produce a shearing interferogram which in turn is examined, to determine the changes in the slope of the wavefront of the incoming beam. The relative motion of the gratings with respect to the incoming wavefront produce electrical A.C. signals within the photodiode arrays, the signals are in turn detected for relative phase shift to indicate the slope of the wavefront in a given direction. The process is repeated in a direction generally at right angles to the previous motion of the gratings, and the X and Y wavefront slopes may then be employed to reconstruct the wavefront shape.
It is highly desirable to measure the shape of the wavefront of an incoming pulsed beam of light of short duration. Under these circumstances, the use of an A.C. shearing interferometer taught in the prior art may not be practical since the wavefront in the case of a pulsed beam may not have a sufficient time duration to be examined in accordance with the principles of A.C. interferometry. What is desired, is a wavefront sensor which maximizes the utilization of the available light in the short time interval of reception of the pulsed beam. It is also highly desirable to provide a wavefront sensor which has a high photon efficiency whereby virtually all of the light is utilized to make the phase estimates required to produce the wavefront slope information. It is also highly desirable to provide a device which is highly stable and may be fabricated in a compact manner by means of a monolithic block of optical material of small size. It is a further object of the invention to provide a wavefront sensor which has a broad wavelength band capability with a near zero optical path difference throughout.